1. Field of the Invention
The present invention relates to multi-color offset printing presses, for variable formats, intended for printing on continuous webs from rollers or fan-folded sheets and perforated such as `Caroll` types.
2. Description of the Prior Art
At the present, continuous printing is carried out on very large printing units which are well adapted with high rates for heavy runs. However, the setting up is long and out of proportion for short or medium runs. As well as this, the rapid development of microcomputing and falling production costs mean that many medium-sized companies are becoming computerized. This has created a new demand for short and medium runs of continuous printing. Traditional printers, equipped mainly with sheet-fed presses, cannot respond.
Producers of offset presses, aware of this new market, are beginning to produce, in relatively small numbers, equipment which is more appropriate to this demand. However, their lack of versatility added to the fact that their prices, compared to similar sheet-fed equipment, are about three times as much, make it difficult to absorb their cost at the present stage of development of this new procedure. On top of this, these presses are only designed for the printing of forms or labels for computer use. These presses cannot deal with continuous web printing where there are no `Caroll` perforations. Their traction systems for the web and their margin guide are related to the holes of these perforations.
Now in the field of printing from an unperforated web for short or medium runs, such as labels for example, there exists a massive market which is totally beyond the reach of the traditional printer who only has sheet-fed offset presses.
As well as this, these small presses may have only a mono-color passage through the machine. Sometimes they are able to deal with only one color at each passage through the machine. They may also be equipped with a complementary inking head allowing for an extra color during the same passage. This extra color, which is obtained on the same blanket as that which equips the main printing unit of the press, does not guarantee a superimposition of colors. There is a risk of the inks running together after a short run, even by using inks of a different viscosity and taking all sorts of precautions.
For a professional printer, these complementary inking heads leave their particular color on a single blanket which is inked with another color by the inking unit of the press. They are very useful for printing which does not require a perfect juxtaposition of colors. It happens that at the moment the majority of continuous printing requires at least two or three colors of which some are superimposed. This necessitates the use of a blanket per color to be sure of obtaining, without problems, a perfect job.
These problems are the same for continuous printers who don't have either one or several color presses enabling them to change the format, at will and in a few minutes, depending on the different types of successive printing materials to be dealt with. Professional continuous printers at present are equipped with multi-color presses where each printing unit has its own printing blanket, thus avoiding the risk of different colored inks running together. The rates of these presses are in the order of 30,000 to 50,000 copies per hour.
A prior art press is shown in FIG. 1. A roll of paper feeds the press 2 with a web of paper 3 which goes successively through an automatic lateral guiding device 4, then between the blanket bearing cylinders 5 and the pressure cylinders 6 of the different printing units 7. Having been printed, the paper web 3 is then drawn by a rotary press 8 and rerolled with the help of the roll holder 9 or treated with the help of established devices. These devices are adaptable depending on their use and enable, for example, `Caroll` type perforations to be obtained, then longitudinal and transversal perforations and then fan-folding. Depending on the presses and the end product, cutting devices are attached at the exit of the press to change the web into sheets. These machines are very efficient for long runs but in the case of short or medium runs (of the order of 1000 to 20,000 copes) which succeed each other with variable formats, they are inadequate. They require too much precise setting time, usually requiring several hours in the case of a change of format. They have only a limited number of formats.
This comes mainly from their operating principle. In effect, these presses work continuously without stopping between each printing cycle. The circumference of the blanket-bearing printing cylinders 5, the pressure cylinders 6 and the plate-bearing cylinders 10 determine the duration of the printing and therefore the format. This means that for each change of printed material which isn't of the same format or could be divided from it, it is necessary to change the cylinders and the sets of gears controlling them. The printer is obliged to possess and change on each press a certain number of cylinders of variable diameters. In any case because of this requirement, the printer is very limited by the formats of the printed materials he can offer to his clients.
The designers of continuous printing presses, aware of this problem, tend to propose to the users continuous presses with variable formats. In the prior art type of FIG. 1, the printing units 7 are interchangeable, which saves some of the time. There nevertheless is still too much time required between each setting occasioned by a change of format. This solution is costly and limiting for the printer. This solution is reflected in the price of the printed material and is only a palliative which seriously limits the number of possible formats.
Recently a three-color continuous press, with a variable format, and only dealing with `Caroll` fan-folded form webs, has brought the beginning of a solution to this problem. However, it has, amongst others, two serious drawbacks. The three colors are obtained from a single blanket. It has only been designed to deal with forms from `Caroll` webs. As well as this, the basic design means that it is not possible to build units which can be distributed widely at a low price.